C++Talk.NET C++ language newsgroups   Archives   FAQ   Search   Memberlist   Usergroups   Register   Profile   Log in to check your private messages   Log in  the meaning of lvalue in C++          C++Talk.NET Forum Index -> C++ language, library and standards View previous topic :: View next topic   Author Message restor Guest Posted: Fri Mar 09, 2007 8:25 pm    Post subject: the meaning of lvalue in C++ Does lvalue have any useful meaning in C++? The original (i.e.: as used in C) was that it is whatever that can be assigned a value. But in C++ the code: std::complex<double> Complex() { return std::complex<double>(); } Complex() = Complex(); Is valid. Also the code: int Integer() { return 1; } const int& cref = Integer(); int& ref = const_cast<int&>( cref ); // is it a cast from rvalue to lvalue??? ref = 3; Is valid, but it is unclear (at least for me) what is 'ref'. Is it an lvalue pointing to a temporary? We also have non-modifiable lvalues (consts) and modifiable rvalues like here: ReturnObj().ModifyMe(); So what is useful in having a concept of lvalue in C++? --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top jam Guest Posted: Fri Mar 09, 2007 11:26 pm    Post subject: Re: the meaning of lvalue in C++ On Mar 10, 12:25 am, "restor" <akrze...@interia.pl> wrote: Quote: Does lvalue have any useful meaning in C++? The original (i.e.: as used in C) was that it is whatever that can be assigned a value. But in C++ the code: std::complex<double> Complex() { return std::complex<double>(); } Complex() = Complex(); Is valid. Also the code: int Integer() { return 1; } const int& cref = Integer(); int& ref = const_cast<int&>( cref ); // is it a cast from rvalue to lvalue??? ref = 3; Is valid, but it is unclear (at least for me) what is 'ref'. Is it an lvalue pointing to a temporary? We also have non-modifiable lvalues (consts) and modifiable rvalues like here: ReturnObj().ModifyMe(); So what is useful in having a concept of lvalue in C++? What is the use of an lvalue by the way? I should say that even const objects can be lvalue: struct Lvalue{ Lvalue operator=(const Lvalue &)const{};//look at this!! }; { const Lvalue L1,L2; L1=L2;//huh huh look at me }; generally the only real Rvalue in C++ are literal numbers and enumerations. Have fun FM --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top James Kanze Guest Posted: Mon Mar 12, 2007 6:00 pm    Post subject: Re: the meaning of lvalue in C++ On Mar 9, 9:25 pm, "restor" <akrze...@interia.pl> wrote: Quote: Does lvalue have any useful meaning in C++? The original (i.e.: as used in C) was that it is whatever that can be assigned a value. Except that that use was quickly violated in C as well, since you can have const lvalues. Quote: But in C++ the code: std::complex<double> Complex() { return std::complex<double>(); } Complex() = Complex(); Is valid. The value of lvalues is somewhat (but not completely) attenuated for class types. Quote: Also the code: int Integer() { return 1; } const int& cref = Integer(); int& ref = const_cast<int&>( cref ); // is it a cast from rvalue to lvalue??? No. cref is an lvalue to begin with. Quote: ref = 3; Is valid, No it's not. It's undefined behavior. You can get similar situations in C: int const i = 3 ; int const* p = &i ; int* q = (int*) p ; *1 = 4 ; Also undefined behavior (both in C and in C++). Quote: but it is unclear (at least for me) what is 'ref'. Is it an lvalue pointing to a temporary? It's a non-const reference to an int. It's also an lvalue. And yes, it happens to designate a temporary; that's something that I don't think you can do in C. On the other hand, as long as the temporary is alife, why not? The real problem is that you can get lvalue expresions (both in C and in C++) which refer to objects which no longer exist. Which results in undefined behavior in both languages. Quote: We also have non-modifiable lvalues (consts) and modifiable rvalues like here: ReturnObj().ModifyMe(); So what is useful in having a concept of lvalue in C++? Mostly, I think, for the non-class types. It's mainly relevant in exprssions, and of course, about the only thing you can do with a non-class types in an expression is call functions (on it or with it as an argument). But it still places a small role with class types: &Class() isn't legal, for example, and you cannot bind an rvalue directly to a non-const reference. -- James Kanze (Gabi Software) email: james.kanze (AT) gmail (DOT) com Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top Guest Posted: Mon Mar 12, 2007 7:19 pm    Post subject: Re: the meaning of lvalue in C++ On Mar 9, 11:26 pm, "jam" <farid.mehr...@gmail.com> wrote: Quote: On Mar 10, 12:25 am, "restor" <akrze...@interia.pl> wrote: Does lvalue have any useful meaning in C++? The original (i.e.: as used in C) was that it is whatever that can be assigned a value. But in C++ the code: std::complex<double> Complex() { return std::complex<double>(); } Complex() = Complex(); Is valid. Also the code: int Integer() { return 1; } const int& cref = Integer(); int& ref = const_cast<int&>( cref ); // is it a cast from rvalue to lvalue??? ref = 3; Is valid, but it is unclear (at least for me) what is 'ref'. Is it an lvalue pointing to a temporary? We also have non-modifiable lvalues (consts) and modifiable rvalues like here: ReturnObj().ModifyMe(); So what is useful in having a concept of lvalue in C++? What is the use of an lvalue by the way? I should say that even const objects can be lvalue: struct Lvalue{ Lvalue operator=(const Lvalue &)const{};//look at this!! }; { const Lvalue L1,L2; L1=L2;//huh huh look at me }; generally the only real Rvalue in C++ are literal numbers and enumerations. Also: - non-reference return types (including some built-in operations, such as ! and unary &) - explicitly constructed temporaries (e.g. T(3), where T is a type constructible from an int) - the 'this' pointer James --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top James Kanze Guest Posted: Tue Mar 13, 2007 2:47 pm    Post subject: Re: the meaning of lvalue in C++ On Mar 10, 12:26 am, "jam" <farid.mehr...@gmail.com> wrote: Quote: On Mar 10, 12:25 am, "restor" <akrze...@interia.pl> wrote: Does lvalue have any useful meaning in C++? The original (i.e.: as used in C) was that it is whatever that can be assigned a value. But in C++ the code: std::complex<double> Complex() { return std::complex<double>(); } Complex() = Complex(); Is valid. Also the code: int Integer() { return 1; } const int& cref = Integer(); int& ref = const_cast<int&>( cref ); // is it a cast from rvalue to lvalue??? ref = 3; Is valid, but it is unclear (at least for me) what is 'ref'. Is it an lvalue pointing to a temporary? We also have non-modifiable lvalues (consts) and modifiable rvalues like here: ReturnObj().ModifyMe(); So what is useful in having a concept of lvalue in C++? What is the use of an lvalue by the way? I should say that even const objects can be lvalue: Even in C. Quote: struct Lvalue{ Lvalue operator=(const Lvalue &)const{};//look at this!! }; { const Lvalue L1,L2; L1=L2;//huh huh look at me }; generally the only real Rvalue in C++ are literal numbers and enumerations. And most expressions. What's particular about class type temporaries isn't that they aren't rvalues; it's that it's possible, indirectly, to get their address (via the this pointer) even though they are rvalues. As for the "utility": I think it's largely a question of C compatibility. Not just at the language level, but at a larger "look and feel" level. One could easily do away with the concept entirely, allowing things like "&(i + 3)" (with type const int? and of course, the usual lifetime of a temporary), but somehow, the language wouldn't feel the same. -- James Kanze (GABI Software) email:james.kanze (AT) gmail (DOT) com Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top restor Guest Posted: Wed Mar 14, 2007 9:45 pm    Post subject: Re: the meaning of lvalue in C++ So, would it be correct to say, that rvalue is either a literal (or something literal-like: enum, initializer-list) or a temporary? &rzej --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top James Kanze Guest Posted: Thu Mar 15, 2007 3:49 pm    Post subject: Re: the meaning of lvalue in C++ On Mar 14, 10:45 pm, "restor" <akrze...@interia.pl> wrote: Quote: So, would it be correct to say, that rvalue is either a literal (or something literal-like: enum, initializer-list) or a temporary? More or less, maybe. My own position is simply that rvalue and lvalue are arbitrary categories, with no real meaning outside of the standard's specification of expressions, where it is explicitly stated whether the results of each expression are an rvalue or an lvalue, and whether the expression requires an rvalue or an lvalue. (There is an lvalue to rvalue conversion, so an lvalue can always be used where an rvalue is required.) In the end, today, it can be argued that the distinction is nothing more than a way of artificially constraining expressions. But such constraints have a historical origin, and participate in the "look and feel" of the language; allowing things like &3 or &(i+2) just doesn't "feel" right. Even though there are situations where such expressions do have an address, and it is possible to obtain it. -- James Kanze (GABI Software) email:james.kanze (AT) gmail (DOT) com Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top jam Guest Posted: Sat Mar 17, 2007 4:00 pm    Post subject: Re: the meaning of lvalue in C++ On Mar 12, 10:00 pm, "James Kanze" <james.ka...@gmail.com> wrote: Quote: On Mar 9, 9:25 pm, "restor" <akrze...@interia.pl> wrote: Does lvalue have any useful meaning in C++? The original (i.e.: as used in C) was that it is whatever that can be assigned a value. Except that that use was quickly violated in C as well, since you can have const lvalues. But in C++ the code: std::complex<double> Complex() { return std::complex<double>(); } Complex() = Complex(); Is valid. The value of lvalues is somewhat (but not completely) attenuated for class types. Also the code: int Integer() { return 1; } const int& cref = Integer(); int& ref = const_cast<int&>( cref ); // is it a cast from rvalue to lvalue??? No. cref is an lvalue to begin with. ref = 3; Is valid, No it's not. It's undefined behavior. You can get similar situations in C: int const i = 3 ; int const* p = &i ; int* q = (int*) p ; *1 = 4 ; Also undefined behavior (both in C and in C++). but it is unclear (at least for me) what is 'ref'. Is it an lvalue pointing to a temporary? It's a non-const reference to an int. It's also an lvalue. And yes, it happens to designate a temporary; that's something that I don't think you can do in C. On the other hand, as long as the temporary is alife, why not? The real problem is that you can get lvalue expresions (both in C and in C++) which refer to objects which no longer exist. Which results in undefined behavior in both languages. We also have non-modifiable lvalues (consts) and modifiable rvalues like here: ReturnObj().ModifyMe(); So what is useful in having a concept of lvalue in C++? Mostly, I think, for the non-class types. It's mainly relevant in exprssions, and of course, about the only thing you can do with a non-class types in an expression is call functions (on it or with it as an argument). But it still places a small role with class types: &Class() isn't legal, for example, and you cannot bind an rvalue directly to a non-const reference. -- James Kanze (Gabi Software) email: james.ka...@gmail.com Conseils en informatique orientée objet/ Beratung in objektorientierter Datenverarbeitung 9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34 --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-...@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ:http://www.comeaucomputing.com/csc/faq.html ] Except literals nothing should be constreined to be none changing.Liberity is a divine gift.I do not see any thing wrong with this in C++. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. If that fails, use mailto:std-c++@ncar.ucar.edu ] [ --- Please see the FAQ before posting. --- ] [ FAQ: http://www.comeaucomputing.com/csc/faq.html ] Back to top David Thompson Guest Posted: Sun Apr 15, 2007 7:40 pm    Post subject: Re: the meaning of lvalue in C++ On Mon, 12 Mar 2007 12:00:58 CST, "James Kanze" <james.kanze (AT) gmail (DOT) com> wrote: Quote: On Mar 9, 9:25 pm, "restor" <akrze...@interia.pl> wrote: Does lvalue have any useful meaning in C++? The original (i.e.: as used in C) was that it is whatever that can be assigned a value. In BCPL, the primary ancestor of C, which had/modelled only uniformly writable storage, it was simply any storage address. Quote: Except that that use was quickly violated in C as well, since you can have const lvalues. Not so quick, about 15 years. But arrays were unassignable lvalues from the beginning of C -- as opposed to BCPL and IIRC B where they were truly just pointers, as many people still wrongly believe and say for C and (perhaps less so) C++. --- [ comp.std.c++ is moderated. To submit articles, try just posting with ] [ your news-reader. 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